Projection systems use a lens or system of lenses to project an image of an image source onto a viewing screen. The image source displays the information to be projected. If the image on a cathode-ray tube ("CRT") is used as an image source, as in a projection television system, light emitted by the CRT is collected and projected onto the viewing screen. Other projection systems, such as overhead projectors, use an image source that is not luminescent and, therefore, require a light source separate from the image source. A common image source used with an overhead projector is a thin plastic sheet having transparent and opaque areas. The sheet typically rests on a stage, and light from a light source below the stage is projected through and attenuated by the sheet before reaching the viewing screen. In a projector that uses a liquid crystal display ("LCD") panel, areas of transparency and opacity can be controlled electronically by a computer to display a series of images. LCD projectors include those constructed with a built-in light source and LCD, as well as projectors in which a separate LCD panel is placed on a stage as an image source.
FIG. 1A shows a typical LCD projector 10 that includes a light source 12 producing light 14 that is collimated by a collimating lens 16. Light 14 passes through, and is attenuated by, an LCD 18 that is displaying an image to be projected. Light 14 next passes through a field lens 30 that concentrates light 14 from LCD 18 into a projection lens 32, which forms an image 40 on a viewing screen 42 of the image displayed on LCD 18. The optical components of LCD projector 10 are also shown in FIGS. 2A, 3A, and 4A for describing other prior art LCD projectors.
Projector 10 is typically positioned on a tabletop. It is generally desirable to position the image with its center 44 above the level of the projector to allow viewers to see an unobstructed image. Methods used to position an image above the level of the projector can cause distortion.
If an optical axis 46 of the projection lens 32 is perpendicular to the image source 18 and viewing screen 42, as shown in FIG. 1A, an undistorted image is projected onto viewing screen 42. FIG. 1B shows an undistorted image 40 of a rectangle by the system of FIG. 1A. If the image is projected at an upward angle 50 onto screen 42 so that the center of the image is above the level of the projector, as shown in FIG. 2A, a distorted image 52 (FIG. 2B) is formed on viewing screen 42. Such a distorted image 52 is formed on viewing screen 42 when optical axis 46 of the projection lens 32 is not perpendicular to both the LCD 18 image source and the viewing screen 42. FIG. 2B shows that image 52 of a rectangle projected by the system shown in FIG. 2A appears as a trapezoid or keystone. This distortion phenomenon is called, therefore, "keystone distortion" or "keystoning."
Keystoning occurs because the distance 54 from projection lens 32 to an upper edge 56 of image 52 is greater than the distance 72 from projection lens 32 to a lower edge 74. The magnification of image 52, which is proportional to the distance from the plane of the projection lens to image plane is, therefore, greater at upper edge 56, resulting in a square image source producing a keystone-shaped image. It will be understood that, although overhead projectors typically use a mirror to change the projection direction, the above analysis still applies; optical axis 46 of the projection lens 32 is considered to be bent by the mirror in the same manner as the light forming the image.
When a CRT-generated image is used, keystoning can be reduced by predistorting the image source on the CRT such that when keystoning changes the image shape, the projected image appears undistorted. Predistortion requires deforming the image source with a magnification equal and opposite to that of the keystone distortion. Such deformation is possible with a CRT-generated image source because the continuous phosphor layer in a CRT provides pixels lacking discrete boundaries. On an LCD, however, the pixels have more sharply defined boundaries and magnification variations required to correct keystoning would cause vertical lines in the image to have a jagged appearance. Predistortion could possibly be implemented in an LCD projector using pixels of nonuniform sizes to reduce the jagged appearance, but such displays are expensive to manufacture and eliminate keystone distortion at only a particular projection angle.
A method of positioning the center of an image above the level of the projector without causing keystone distortion is described in U.S. Pat. No. 4,436,393 to Vanderwerf for "Distortion Correction for an Overhead Projection System." This method, hereinafter referred to as the "offset method," requires a projection lens 76 having an image field larger than that of projection lenses in other projectors. Projection lens 76 has an optical axis 78, and LCD image source 18 has a center 80. The offset method entails projecting image 40 perpendicularly to LCD image source 18 and the viewing screen 42, but offsetting center 80 of LCD image source 18 away from optical axis 78, as shown in FIG. 3A. This method is analogous to projecting, in the manner shown in FIG. 1, a large image that extends above and below the level of the projector 10 and then blocking the lower portion of the image; the remaining portion of the image extends above the level of projector 10 and exhibits no keystoning. Because the offset method uses only a portion of the image field of projection lens 76 to project image 40, the method requires a more expensive projection lens with a larger image field than standard projectors require.
Another method of positioning an image above the level of the projector without keystoning, hereinafter referred to as the "tilted field lens" method, entails tilting a top edge 82 of field lens 30 towards projection lens 32 as shown in FIG. 4A, thereby changing the relative magnification of portions of the image. This method is used in the "Litepro" LCD projector from In Focus Systems, Inc. of Tualatin, Oregon. FIG. 5A is an image exhibiting keystone distortion formed 84 by a computer generated ray tracing program modeling the system of FIG. 2A. FIG. 5B is an image 86 formed by the computer generated ray tracing program modeling the system of FIG. 4A that uses a tilted field lens. FIG. 5B shows that tilting the fresnel lens eliminates the keystoning.
The methods described above are effective for reducing keystone distortion in systems that use a single projection lens for projecting the image. Some systems, however, use multiple projection lenses, each of which projects an image. Each projection lens is part of a train of optical elements that also includes a field lens and may include a separate image source. Multiple images from the multiple projection lenses are typically superimposed to appear as a single image.
For example, a three projection lens, multicolor system typically includes a central projection lens and two side projection lenses, one on either side of the central projection lens. Each lens projects an image in a different primary color, and the three images converge to produce a single full-color image. In a three-lens system, the images from each of the three lens must be properly focused on the viewing screen and converged, i.e., superimposed onto the images of the other lens to produce a correct image.
The side lenses project their images at a slight angle toward the center image so that the images overlap. Light from the side lenses, therefore, strikes the viewing screen at a nonperpendicular angle and keystoning results. Such keystoning is referred to as "horizontal" keystoning to distinguish it from "vertical" keystoning described above as caused by projecting an image at an angle above the level of the projector. Although horizontal keystoning can be corrected by the methods described above for correcting vertical keystoning, correcting both horizontal and vertical keystoning simultaneously presents a problem.
Correcting both horizontal and vertical keystoning by offsetting the center of the image source from the optical axis of the projection lens would position the image source farther off-center in the image field of the projection lens than would correcting keystoning in only one direction. For the image field of the projection lens to encompass the offset image source, an expensive projection lens with a large image field is required.
Correcting vertical keystoning by the field lens tilt method and simultaneous correcting horizontal keystoning by the offset method results in another type of distortion known as "trapezoidal" error. Depending upon the arrangement of optical components, trapezoidal error can appear as a trapezoid or as a parallelogram, with the top and bottom of the image of the correct size but displaced sideways with respect to each other.
FIG. 6A shows a parallelogram-shaped image 90 formed by projecting light 14 from a square image source through a projector 10 that uses a tilted field lens 30 to eliminate vertical keystoning and an offset image source 18 to eliminate horizontal keystoning. FIG. 6B shows image 90 together with a second image 92 that exhibits trapezoidal error in a sense opposite to that of image 90. Images 90 and 92 are formed when the optical axes of the two projection lenses intersect on the viewing screen 42. Trapezoidal error prevents images 90 and 92 from converging at points away from a center line 94 of the images; the two images are properly converged only at points along center line 94. Such lack of convergence causes blurred images and incorrect colors in multilens systems.